Total hip arthroplasty refers to the replacement of natural hip joint components with artificial ball and socket devices of various complexity and configurations. A generally spherical ball comprised of metallic or ceramic materials is often attached to an artificial femoral stem implanted in the femur, while a generally hemispherical socket is implanted into a surgically modified acetabulum.
Artificial hip joint designs are typically classified according to the paired materials employed in the prosthesis. In “metal-on-metal” designs, both ball and socket are comprised of various metallic alloys (such as stainless steel, CoCr, and CoCrMo), as exemplified in U.S. Pat. No. 3,848,272 to Noiles and U.S. Pat. No. 7,361,194 to Carroll. In “ceramic-on-metal” or “ceramic-on-ceramic” designs, a ceramic ball (typically comprised of alumina or zirconia) is attached to a metallic or ceramic femoral stem while the socket is comprised of ceramic materials or various metallic alloys (such as stainless steel, CoCr, or CoCrMo), as exemplified in U.S. Pat. No. 6,881,229 to Khandkar et al.; U.S. Pat. No. 5,788,916 to Caldarise; and U.S. Pat. No. 3,924,275 to Heimke et al. In “metal-on-plastic” or “ceramic-on-plastic” designs, the ball is comprised of either metallic or ceramic materials while the socket is comprised of a plastic hemispherical cup (typically comprised of ultra-high-molecular-weight polyethylene (UHMWPE)) which is attached in various ways to a metallic shell, as exemplified in U.S. Pat. No. 5,080,677 to Shelley.
A continuing problem with current artificial hip joint replacements is wear of the articulating surfaces. In metal-on-plastic designs, wear particles from the relatively softer UHMWPE material can generate an autoimmune reaction in the body known as osteolysis which results in resorption of living bone tissue surrounding the artificial socket and subsequent loosening or detachment of the socket from the acetabulum. In metal-on-metal designs, high concentrations of metallic ions associated with nanoscale wear particles have been found deposited in the surrounding tissue, and these high ion concentrations may pose long-term health concerns.
Aggravated wear is linked to the lack of full-film lubrication prevalent with current artificial hip joint designs. Synovial fluid is generated in body tissues surrounding the artificial hip joint. The load transmitted from ball to cup varies in magnitude and direction but does not reverse direction during the gait cycle. Thus, the only mechanism then capable of supplying synovial fluid to the joint is “wedge-film” action generated by relative tangential surface motions associated with the gait cycle kinematics. Even though ball and cup contacting surfaces are conformal, the ball and cup elastic properties, load magnitude, and the surfaces' radii of curvature result in a load-carrying lubricated contact region which covers only a small percentage of the total possible surface contact area. Elastohydrodynamic analysis methods appropriate for such locally lubricated contacts predict minimum film thickness values on the order of 40 to 60 nm (Mattei et al., “Lubrication and Wear Modelling of Artificial Hip Joints: A Review,” Tribology International 44:532-549 (2011)) which when compared with surface roughness values are generally in the boundary to mixed-lubrication regime. Contacting surfaces operating in these regimes of lubrication are not completely separated, resulting in surface wear generated by either direct contact of surface asperities (adhesion wear) or through wear particles wedged between the surfaces (abrasive wear). Compounding the lubrication and wear problem is the observance of relatively thick protein layers which are in suspension in the synovial fluid and which are accumulated on the articulating surfaces in the contact region. These protein layers can be on the order of 100 nm thick (Sprecher et al., “Solid Lubrication—A Relevant Lubrication Mechanism for Reducing Wear in Metal-on-Metal THA Components?” In 49th Annual Meeting, ORS, p. 1391 (2003)).
Manufacturing and fabrication concerns are also prevalent with current artificial hip joint designs. Ceramic-on-ceramic designs are prone to squeaking during walking, presumably due to stick-slip friction developed between the articulating surfaces (Feder “That Must Be Bob. I Hear his New Hip Squeaking,” The New York Times, May 11, 2008 (2008)). Ceramic-on-ceramic designs are also relatively more expensive and brittle in nature, so particular attention is needed for both fabrication and surgical procedure.
In U.S. Pat. No. 5,609,646 to Field et al., an artificial acetabular component is comprised of an outer reinforcing backing and an inner bearing component, the latter of which has two independent protruding arms. The intent of this arrangement is to provide adequate flexibility and accommodate deformation of the natural portion of the acetabulum under loading. In U.S. Patent Application Publication No. 2009/0259317 to Steinberg, an elastic socket insert is described. U.S. Pat. No. 6,248,132 to Harris describes an interior spring assembly placed between the outer shell and inner cup which acts as a shock absorber during the load phase of the gait cycle. In U.S. Pat. No. 5,788,916 to Caldarise, a set of leaf springs are formed in the outer metallic shell which also acts as a shock absorber during the load phase of the gait cycle. In U.S. Patent Application Publication No. 2001/0051831 to Rao et al. and U.S. Pat. No. 5,389,107 to Nassar et al., an interior spring and shock absorbing materials are connected between the ball and femoral stem. These patents and patent applications all assume that the ball and cup surfaces will come into contact with each other, and the structural elasticity described in these patents is employed to mitigate impact damage and dampen stress waves induced from contact. None of these documents pertain to squeeze-film fluid action to keep the load-carrying surfaces apart, nor do they provide an elastic spring contact connection between ball and cup to intentionally separate the surfaces during the (unloaded) swing phase of the gait cycle.
In U.S. Pat. No. 5,879,386 to Jore, natural magnetic materials inserted into the bone provide repelling forces between articulating joints. U.S. Patent Application Publication No. 2002/0087213 to Bertram employs magnetic materials to provide repelling forces between the ball and cup in order to stabilize hip joint motion. Neither of these documents provides a mechanical means to separate the surfaces in the manner described in the present invention.
In GB Patent No. 1,192,555 to New, a set of two struts provide a connection between the wristpin and connecting rod in a cylindrical bearing arrangement appropriate for two-stroke engines where the external piston load is variable in magnitude but does not reverse direction. These struts provide a mechanical means of separating the wristpin bearing surface from the small-end sleeve surface when the external load magnitude is minimal over the engine cycle. However, the particular design of the strut geometry and the interaction of the strut with hemispherical contact surfaces built into the wristpin restrict this embodiment to cylindrical journal bearing geometry where journal translation (relative to sleeve) and external loads are planar (two-dimensional) and where the rotation of the sleeve relative to the journal is about a single axis normal to the plane. In addition, GB Patent No. 1,192,555 to New places the positioning of the mechanical spring mechanism external to the bearing region.
In Meng et al., “Contact Mechanics and Elastohydrodynamic Lubrication in a Novel Metal-on-Metal Hip Implant with an Aspherical Bearing Surface,” Journal of Biomechanics 43:849-857 (2012); European Patent No. 0748193 to Fisher; and PCT Publication No. WO/1995/023566 to Fisher, non-spherical ball and cup articulating surfaces are described. However, none of these documents pertain to squeeze-film fluid action to keep the load-carrying surfaces apart nor do these documents provide an elastic contact connection between ball and cup to intentionally separate the surfaces during the (unloaded) swing phase of the gait cycle.
The present invention is directed to overcoming deficiencies in the art.